Water
Water tank and rainwater calculator
This calculator answers two questions. How big a storage tank does a household need to get through days without council water? And how much rainwater could the roof collect over the year?
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
Storage depends on how many people there are, how much each uses and how many days the tank must last. Around 100 litres per person per day covers drinking, cooking, washing and flushing; a garden or a swimming pool needs far more. Tanks come in standard sizes, so the calculator rounds up to the tanks you would actually buy.
Harare receives roughly 800 mm of rain a year, almost all of it between November and March. Every square metre of roof catches about 0.8 litres for every millimetre of rain once gutters and first-flush losses are allowed for, so even a modest roof collects a lot of water.
How it is worked out
- Storage = people × litres per person per day × days.
- Tanks are chosen from standard sizes of 1,000, 2,500, 5,000 and 10,000 litres.
- Rainwater per year = roof area × rainfall × runoff coefficient, in litres.
What it assumes
- Daily use does not include gardens, livestock or pools.
- Rainfall is an annual average; individual years vary a lot.
Common questions
What size water tank for a family of five?
At 100 litres per person per day and three days of storage, 1,500 litres; a 2,500 litre tank gives comfortable margin.
How much rainwater can I collect from my roof?
About 0.8 litres per square metre per millimetre of rain. A 150 m² roof in Harare (about 800 mm a year) can collect roughly 96,000 litres a year.
Where should the tank go?
High enough to feed the house by gravity, or with a booster pump, on a base designed for the weight: 5,000 litres of water weighs five tonnes.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.